1. Field of the Invention
The present invention relates generally to electroacoustic transducers such as loudspeakers and, more particularly, to flat plate loudspeakers having two parallel sheet diaphragm members sandwiched about an intermediate honeycombed core.
2. Description of the Prior Art
In conventional flat-plate loudspeakers a pair of diaphragms in the form of sheet members are sandwiched about a honeycomb core structure. The speaker drive piston or voice coil bobbin with a coil wound thereon is connected at one end to the rear sheet diaphragm member. Thus the voice coil bobbin imparts vibrations to the rear sheet diaphragm member. The vibrations imparted to the rear sheet diaphragm member are in turn imparted to the front sheet diaphragm member through the piston effect of the vibrating rear sheet diaphragm member acting against the air within the honeycomb core structure.
As is well known in loudspeakers of this general type, the honeycomb core structure has a high plane stiffness as a whole but the front and rear sheet diaphragm members do not. Therefore, in operation these conventional flat plate loudspeakers when driven by the voice coil bobbin bend the rear sheet diaphragm member slightly near the portion of the rear sheet member where the voice coil bobbin is connected.
Since this bend is very slight, it cannot be found in the frequency response or distortion response of the loudspeaker. However, the bending imparted to the rear sheet diaphragm member is of sufficient magnitude to introduce an extraneous and unwanted auditory sound and thus decreases the quality of the sound reproduced by the loudspeaker.
This deleterious effect is also compounded by the fact that there is a delay between the time vibration is imparted to the rear sheet diaphragm member by the voice coil bobbin and the corresponding vibration is imprarted to the front sheet diaphragm member due to the time it takes for the air within the honeycomb core to resonantly radiate to transmit vibration to the front sheet diaphragm member. The result is the transmission, due to bending of the rear sheet diaphragm member where it is connected to the voice coil bobbin, of an extraneous auditory sound, for example a booming sound.
However, it has been determined, according to the present invention, that this deleterious drawback of conventional flat plate loudspeakers can be eliminated by coupling the front sheet diaphragm member directly to the voice coil bobbin.